Why Does my AC Smell Musty or Like Mould

A musty or mouldy odour from a split system is almost always a sign of moisture and organic build up inside the indoor unit. Dust and humidity settle on the evaporator coil and drip tray, creating the perfect environment for mould. Each time the fan runs, that smell is pushed into your room.

This guide explains what causes the odour, the quickest way to clear it and how to stop it returning with regular maintenance.

Common causes of a musty AC smell

• Dirty filters that trap dust and stay damp
• Mould on the evaporator coil and in the drip tray
• Blocked drain or kinked drain hose leaving stagnant water in the unit
• Dust and biofilm on the blower wheel and behind the louvres
• Long idle periods in cool, humid rooms that encourage microbial growth
  
  

If you have recurring smells, it is a strong sign the unit needs a thorough internal clean, not just a filter rinse.

Quick checks to try first

• Remove and rinse the filters with lukewarm water. Let them dry fully
• Make sure the drain hose is not blocked or kinked and that water can flow freely
• Run the unit on fan mode for ten minutes to dry internal surfaces after use
  
  

If the smell persists, clean the coil, fan and drip tray. That is where most odours start.

How to remove the musty smell properly

You can clear odours at the source with a full DIY clean. This restores healthy airflow and removes mould and residue from the parts that cause smells.

1. Turn power off at the isolator or breaker
   
2. Open the front panel and remove the filters
3. Fit a catch bag to protect walls and floors
4. Apply coil cleaner to the evaporator coil, blower and drip tray
5. Brush away loosened grime and mould
6. Rinse thoroughly with clean water and let the unit drain
7. Rinse filters and allow them to dry completely
8. Reassemble, restore power and run on fan mode to finish drying

Everything you need is in the DIY Air Conditioner Cleaning Kit. It includes coil cleaner, brushes, a pump bottle for rinsing, microfibre cloths, a collapsible bucket and a catch bag.
DIY Air Conditioner Cleaning Kit

For the detailed step by step process, follow the official instructions here.
Split System Aircon Cleaning Kit Guide

Health and comfort reasons to fix it now

A musty AC smell is more than unpleasant. It can circulate mould spores and bacteria, aggravate allergies and asthma and leave rooms feeling damp and stale. For a deeper look at the risks and hidden costs of neglect, read
What happens if you don’t clean your AC

If you are dealing with sneezing, congestion or sore throats when the unit runs, this article explains the breathing benefits of regular cleaning
Is your aircon making your allergies worse

How to prevent the smell from coming back

• Clean filters every one to two months
• Do a full kit clean of the coil, fan and drain every three to six months, or before summer and before winter
• After cooling or heating, run fan mode for a few minutes to dry internal surfaces
• Keep the room and return grille free of dust to reduce what reaches the coil
  
• Ensure the drain hose has a continuous fall and is not blocked
  
  

Bottom line

A musty aircon smell means moisture and build up inside the unit. A quick filter rinse helps, but odours return unless you clean the coil, fan and drain. Use the kit and follow the guide to remove the smell at its source and keep your indoor air fresh.